A few years ago we were in Coz staying at La Cieba and four of us spoke to a taxi driver who said he knew where there was a cenote on Coz. We got two taxi's, set up some redundant doubles, loaded them into the cabs and were off.
The cenote was near Chankanab, a little inland. We drove on a dirt road, made a right into a swamp, and a couple of hundred yards in there was a clearing in the green slime. I remember thinking "I'm not diving in there" when we drove up, until we looked in the clearing. There in the water was the entrance to a really great cave. We dove for about an hour (we had plenty of air left, but made an arrangement with the taxi drivers to pick us back up in an hour), depth was only 35' (we went into the lower chute). The upper chute was only in about 15-20'. The water was clear, but brackish in spots. It was fresh water on the roof of the cave, salt water on the floor, and brackish in the middle...VERY COOL!
About 100' into the cave we came across a sign that was put there by the Florida cave diving association saying "100 divers have died in caves just like this one, do not enter unless cave certified". It was a little strange reading that while diving, makes you think too much.
If you can find someone to take you there, it's definitely worth the trouble. Of course there's no surface support available so I recommend diving redundant systems. We did and had no problems.
One in our group did have some problems with the closed in space. They (he and an instructor) were in the top chute and he starting kicking too much and silted it out. Good thing they had a line tied on. I doubt he'll ever dive caves again.
I don't know the exact location, but I remember seeing a map online somewhere that had a dive flag right where the cave was. If I come across it, I'll post a link.
Enjoy....
I found the link:
The cenote is called
Aerolito De Paraiso .